1. Field of the Invention
The present invention relates to a method utilized in a wireless communication system, and more particularly, to a method of handling small data transmission in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (COMP) transmission/reception, uplink multiple-input multiple-output (UL-MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Machine-type communication (MTC) is one type of data communication including one or more entities not requiring human interactions. That is, the MTC refers to the concept of communication based on a network such as the existing GERAN, UMTS, long-term evolution (LTE), or the like used by a machine device instead of a mobile station (MS) used by a user. The machine device used in the MTC can be called an MTC device. There are various MTC devices such as a vending machine, a machine of measuring a water level at a dam, etc. That is, the MTC is widely applicable in various fields. The MTC device has features different from that of a typical MS. Therefore, a service optimized to the MTC may differ from a service optimized to human-to-human communication. In comparison with a current mobile network communication service, the MTC can be characterized as a different market scenario, data communication, less costs and efforts, a potentially great number of MSs for communication, wide service areas, low traffic per MS, etc.
Most MTC devices and several UEs mainly transmit small data (e.g. an IP packet) for specific applications. Such transmission of small data may be regular but infrequent. For the purpose of power saving, the MTC devices and the UEs may enter an idle mode when there is no data required to be transmitted. If the small data needs to be transmitted, the MTC devices or the UEs may exchange a great number of control messages with the network for establishing a radio resource control (RRC) connection, in order to assign data radio bearer (DRB) for transmitting these small data. After the small data transmission is complete, the MTC devices or the UEs may release the RRC connection and the DRB, and return to the idle mode. In such a condition, the wireless communication system may perform a large number of control plane transmissions for only a small number of user plane messages, which causes a waste of radio resources.
In order to solve the abovementioned problem, the 3GPP TS 23.887 has defined several solutions for small data transmission. Such solutions include optimized RRC connection managements (Solutions 1a and 1b), control plane solutions (Solutions 2a and 2b), connectionless approaches (Solutions 3a and 3b), S1/Iu-only optimizations (Solutions 4a and 4b) and keeping the UEs in the connected mode (Solution 5a). However, these solutions may not be feasible in certain conditions. For example, if an eNB without Solution 2a capability receives the RRC connection establishment messages according to Solution 2a, the eNB may get stuck due to reception of an unexpected message. If the small data information contained in the complete message of the RRC connection according to the Solution 3b is invalid, the eNB may need to reject the RRC connection by sending an RRC connection reject message to the UE or the MTC device, but the UE or the MTC device may not receive this RRC connection reject message after the RRC connection is set up. Thus, there is a need for improvement over the prior art.